Automobile fascias, body side moldings (BSM), rockers, etc., are typically produced by an injection molding process followed by painting. The last steps of the painting process require that the painted part be baked for about 30 minutes at, for example, 250° F. This production procedure is proven and functions well. However, there are a number of negatives associated with this process, including: a high scrap rate due to paint defects, expensive tooling costs, burdensome provisions for protection against possible mutilation in handling, and poor stone impact performance in sensitive areas of a motor vehicle (i.e., under highway driving conditions, stones kicked up by other motor vehicles striking certain prone areas of the painted part).
Presently, however, new technologies are developing with the intention of eliminating the high cost of fabricating injection molding tools, and producing parts through the aforementioned injection molding and painting process. These technologies involve, as can be understood by reference to FIG. 1, a laminated thermoplastic sheet 10 which is formed in a thermoforming process. The laminated thermoplastic sheet 10 is composed, for example, of a paint film 12, which may optionally include a paint layer 12a and a clear coat 12b, wherein the color, finish and gloss of the class “A” side of the laminated thermoplastic sheet is matched to that of the paint of the motor vehicle to which the laminated thermoplastic sheet is to be used. An optional removable mask 12c is provided to protect the paint film 12 is removed when the part 20 is completed. The paint film 12 is bonded onto one side of a thermoformable thermoplastic substrate 18, via an adhesive layer 16, wherein the substrate may be, for nonlimiting example, thermoplastic polyurethanes, polyesters, vinyl copolymers, polyvinylchlorides, thermoplastic olefin (TPO), ABS, polyethylene, and blends, copolymers and/or alloys thereof.
Examples of laminated thermoplastic sheets 10 and methods of forming laminated thermoplastic sheets into formed parts 20 are described in U.S. Pat. No. 4,976,896 issued on Dec. 11, 1990 to the assignee hereof, U.S. Pat. No. 4,769,100 issued on Sep. 6, 1988 to the assignee hereof, and U.S. Pat. No. 4,868,030, issued on Sep. 19, 1989 to the assignee hereof; the disclosures of each of said U.S. Pat. Nos. 4,976,896, 4,769,100 and 4,868,030 being hereby herein incorporated by reference. Other U.S. patent references describe additional aspects of laminated thermoplastic sheets and the thermoforming processes therefor, as for example U.S. Pat. Nos. 6,450,793 and 6,709,734 and U.S. Patent Application Publication 2004/0076846.
While the technology for thermoforming laminated thermoplastic sheets has become well established, there yet remains the problem that the thermoforming process adversely affects the gloss of the class “A” finish. For example, the gloss of the class “A” finish may start at a gloss value above 70 but, as a result of thermoforming, the gloss value becomes unacceptably less than 70. Accordingly, in the prior art of thermoforming of laminated thermoplastic sheets, parts have inconsistent finish and gloss, resulting in scrap and/or parts having a finish and/or gloss which does not well match the finish and/or gloss of the paint of conventionally painted surfaces of the motor vehicle.
Thus, there yet remains a need in the art of thermoforming laminated thermoplastic sheets an ability to preserve, reliably under high volume production conditions, the class “A” finish (i.e., the paint film side) of the laminated thermoplastic sheet which well matches the finish of conventionally painted surfaces of a motor vehicle, and retains a gloss value, after thermoforming, of above 70.
The inventors of the present invention have come to the realization that during the processing of the parts, the unique process control required to form a Class “A” part, having critical influence on the gloss of the final part is the absence of spherulite within the paint film.
In the literature, there is reported attempts to control spherulite in PVDF/PMMA blends, as follows.
In the papers “Impact of nucleating agents of PVDF on crystallization of PVDF/PMMA blends”, Polymer 42 (2001), pp. 8799-8806, by S. Schneider, X. Drujon, J. C. Wittman and B. Lotz; and “Self-nucleation and enhanced nucleation of polyvinylidene fluoride (α-phase)”, Polymer 42 (2001), pp. 8787-8798, by S. Schneider, X. Drujon, B. Lotz and J. C. Wittman, the authors discuss the influence of a nucleator to control the size of spherulite size in neat and blended systems. In both of these articles, which articles are hereby herein incorporated by reference, Schneider et al investigate the details of nucleating, but do not demonstrate applicability as is done by this invention.
In the paper “Influence of Additives on the Crystallization Kinetics of Semicrystalline Polymers,” Polymer Engineering and Science, November 2002, Vol. 42, No. 11, by I. Pillin, S. Pimbert and G. Levesque, the authors discuss the influence of organic pigments or mineral fillers and how they influence the non-isothermal crystallization temperatures of PVDF and its blends with PMMA. Again, they discuss the details and specifics of crystallization rate and spherulite formation, but not a method from which to avoid while speeding up the solidification rate without generation of spherulite.
In the paper “Crystallization and Morphology of Melt-Solidified Poly(vinylidene Floride),” from Journal of Polymer Science: Polymer Physics Edition, Vol. 18, pp. 793-809 (1980), by Andrew J. Lovinger, the author discusses the two types of spherulite generated from cooling melted PVDF. The paper simple describes the types of spherulite and how they are generated.
In all the above-referenced papers, it is apparent that the invention is outside the scope of this literature, in that our goal is to prevent or impede formation of spherulite, thus sustaining high gloss.
Accordingly, what remains needed in the art are methods for reducing or eliminating spherulite formation in the paint film of a laminated thermoplastic sheet during a thermoforming process thereof.